It is conventionally known to use a foil bearing as a bearing for a rotating member that rotates at a high speed such as at tens of thousands rpm, in which the foil bearing comprises a plurality of foils (flexible membranes) for forming a bearing surface and supports a journal (or shaft) of the rotating member by means of pressure of a fluid dragged in between the journal and the foils as the rotating member rotates. Some of such foil bearings may comprise a plurality of bump foils as disclosed in U.S. Pat. No. 4,277,113 issued to Heshmat, or may have a plurality of leaf foils as disclosed in U.S. Pat. No. 4,178,046 issued to Silver et al.
In these foil bearings, the bump foils or leaf foils are formed by press-working a flexible thin metallic plate or the like so that they provide a resilient force for urging the journal generally in a direction toward a rotational center. If the foils arranged in the circumferential direction have a uniform rigidity (or stiffness), the center of rigidity coincides with the geometric center of the bearing. In such a case, when the journal is rotating, the rotation center of the journal in an equilibrium state is determined by an amount of deformation of the foils under a weight of the rotating member, and thus, if the rotation is clockwise, the rotation center will be shifted in a lower left direction with respect to the center of the bearing at low rotational speeds.
As the rotational speed of the journal increases, an air force acting upon the journal in an upper left direction becomes larger so that the center of the journal moves clockwise from the lower left shifted position toward the center of rigidity of the foils (or the center of the bearing). When the center of the journal approaches the center of the bearing, the air force acting upon the journal is reduced and the center of the journal moves in the lower left direction again. The process is repeated at a frequency corresponding to the rotational speed to cause a whirling instability of the journal. In case of an air-film foil bearing, a force constraining the journal is relatively small, and thus, the above instability tends to become large particularly at a primary bending resonance point, making it difficult for the rotating member to pass the resonance point safely as the rotation thereof is accelerated.
In order to suppress such an instability and allow the rotating member to pass the primary bending resonance point safely, it is necessary to reduce the amount of deformation of the foils due to the weight of the rotating member, or, to provide the foils with appropriate rigidity and make the foils exert an appropriate damping force. One way for that is to make the center of rigidity of the foils located at a higher position than the geometric center of the bearing by, for example, providing a higher rigidity to the bump foils or leaf foils disposed at lower positions than those disposed at higher positions.
However, it is quite difficult to control with high precision the rigidity or damping characteristics of a number of bump foils or leaf foils which are formed by press-working, and also, change in the rigidity and damping characteristics of the foils requires considerable change in the manufacturing process. Further, arranging the foils such that the foils at various circumferential positions have a suitable rigidity for their positions tends to result in a reduction in the yield of the completed foil bearing which requires quite high precision in various dimensions such as an inner diameter, cylindricity of the inner peripheral surface, etc. Due to these reasons, it has been quite difficult to achieve the center of rigidity of the foil bearing positioned higher than the geometric center of the bearing to reduce the oscillation at the primary bending resonance point and improve the rotation performance of the foil bearing.